Methods and systems for fault isolation and bypass in a dual ring communication system

ABSTRACT

An apparatus and method for fault isolation and bypass in a dual ring communication system provides a reconfiguration unit capable of attachment to a dual ring communication system having a plurality of reconfiguration units. The disclosed reconfiguration unit has a single adapter to the dual ring communication system and includes isolation and wrap switches capable of isolation and bypass of faults. A reconfiguration unit having a single adapter to the dual ring communication system detects failures on the ring and generates a failure frame which if not received from the ring causes the reconfiguration unit to enter a downstream wrap state. The reconfiguration unit enters an upstream wrap state if a failure frame is received from the nearest downstream reconfiguration unit.

RELATED APPLICATIONS

This application is related to concurrently filed and commonly assignedapplication entitled METHODS AND SYSTEMS FOR DETERMINING THE NEARESTDOWNSTREAM RECONFIGURATION UNIT IN A DUAL RING COMMUNICATION SYSTEM,Ser. No. 08/429,452 filed Apr. 27, 1995, now pending.

FIELD OF THE INVENTION

The method and apparatus of the present invention relates generally toserial communication networks and more particularly to dual ringcommunication systems having multiple concentrators or reconfigurationunits.

BACKGROUND OF THE INVENTION

Serial communication networks provide many advantages over otherwell-known networks such as multipoint, star or mesh networks, the mostvaluable being a fair distributed polling function which readilysupports peer-to-peer communications amongst a large number of stationswith a high utilization of the network capacity.

One major drawback of the serial network is its propensity tocatastrophic failure when any component of the network fails. Thedrawbacks of a serial network are somewhat reduced by use of a dual ringcommunication system. A dual ring communication system is comprised of aprimary ring or data path and information is passed in a first directionalong the primary ring. No information is passed along the secondaryring until a failure occurs. When a failure occurs the secondary ringmay be connected to the primary ring to complete the serialcommunication network. In such a case information is transmitted alongthe secondary ring in a second direction opposite that of the primaryring.

Over the years many techniques have been developed to detect and/orisolate faults in serial communication networks. One of the more usefultechniques, which is in use today in the IEEE 802.5 Token Ring, isdisclosed in U.S. Pat. No. 3,564,145. This technique known as beaconingidentifies a station, by its address, immediately downstream of a failednetwork component or station. In a static network (e.g. one in which thenetwork topology is fixed or known) corrective action can be taken tobypass or fix the failed network component.

Another technique (Dual Ring Reconfiguration) has proven very useful inthe isolation of faults in a serial network, thereby providing completeor partial network operation following the failure of a networkcomponent. This technique employs the dual serial rings described abovewhich may be converted to single ring via switching means to therebybypass a failed network component. The patents listed below disclose avariety of Dual Ring Reconfiguration implementations:

U.S. Pat. No. 3,519,750

U.S. Pat. No. 3,876,983

U.S. Pat. No. 4,009,469

U.S. Pat. No. 4,354,267

U.S. Pat. No. 4,390,984

U.S. Pat. No. 4,527,270

U.S. Pat. No. 5,538,264

U.S. Pat. No. 4,594,790

U.S. Pat. No. 4,709,365

The patents listed below disclose a variety of manual and automatictechniques for bypassing a failed network component in a single ringserial network:

U.S. Pat. No. 3,458,661

U.S. Pat. No. 4,035,770

U.S. Pat. No. 4,048,446

U.S. Pat. No. 4,245,343

U.S. Pat. No. 4,763,329

While all of the techniques described above are useful either bythemselves or in combination, they may have difficulty in providingfast, efficient or complete restoration of communications in a serialring network following failures of all kinds.

Modern serial networks such as the IEEE 802.5 Token Ring Networkgenerally employ many (up to several hundred or more) ports locatedthroughout an establishment. Many of these ports are not utilized or areconnected to inactive stations. In addition, stations (each of whichincludes a unique identity or address) are frequently moved from oneport to another for the convenience of the operator.

In view of the mobility of the stations and the large number of portswhich have no or inactive stations connected, the station identity oraddress accompanying a beacon message provides little information tolocate the geographic position of the failed network component.

The technique (Next Active Upstream Neighbor) disclosed in U.S. Pat. No.4,507,777 is very useful in managing fault recovery in serial networks;however, the sequential station identities or addresses derived fromthis technique do not provide sufficient network topology information toaccurately pinpoint the physical location of the failed component. Forexample, two adjacent active stations may be separated on the physicalnetwork by a number of non-connected or inactive ports. Thus, knowingthat station X detected a failure and that station C preceded X does notphysically locate a particular faulty component.

U.S. Pat. No. 5,132,962 describes a reconfiguration unit capable offault isolation and bypass in a dual ring communication system. Whilethis unit fully implements the IEEE 802.5 fault isolation and bypassprocedures, it requires three adapters to interface to the primary andsecondary rings of the dual ring communication system. These adaptersare the single most expensive component of the reconfiguration unit and,therefore, a unit requiring three such adapters may be impractical incertain applications where the high degree of error recovery cannotjustify the cost of such reconfiguration units.

Because many existing dual ring networks have been implemented with thetechnology described above any reconfiguration unit design should becompatible with such networks. Furthermore, to assure compatibility withfuture designs of reconfiguration units it would be desirable that anyreconfiguration unit design or method of fault isolation and bypass becompatible with a network protocol such as the IEEE 802.5 Token Ringstandard.

SUMMARY OF THE INVENTION

In view of the limitations described above, it is an object of thepresent invention to provide a reduced cost reconfiguration unit whichis capable of isolating and bypassing faults on a dual ringcommunication system. Another object of the present invention is toprovide a reconfiguration unit for dual ring communication systemscapable of operation in a communication system having multiplereconfiguration units. It is a further object of the present inventionto provide a reconfiguration unit capable of functioning in existingnetworks having IEEE 802.5 compatible reconfiguration units such as thatdescribed in U.S. Pat. No. 5,132,962.

An additional object of the present invention is to provide a method offault isolation and bypass which may be implemented using areconfiguration unit having only a single adapter to the primary ring ofa dual ring communication system. Another object of the presentinvention is to provide a method of fault isolation and bypass which maybe implemented in networks having multiple adapter reconfigurationunits. Yet another object of the present invention is to provide amethod of fault isolation and recovery which is compatible with the IEEE802.5 Token Ring standard.

In view of the above objectives, one aspect of the present inventionprovides a reconfiguration unit and method of using a reconfigurationunit capable of connection to a dual ring communication system whereinthe reconfiguration unit has a single adapter to the dual ringcommunication system. The reconfiguration unit also has attachment portsfor selectively incorporating devices such as workstations and the likeinto the dual ring communication system. The reconfiguration unitfurther has wrap and isolation switches to selectively wrap the primaryring to the secondary ring of the reconfiguration unit to isolate andbypass faults.

In a particular embodiment of the present invention a method forreconfiguring a dual ring communication system to isolate and bypassfailed components or connections is provided in which the dual ringcommunication system has a primary ring (PR) for carrying signals in afirst direction and a secondary ring (SR) for carrying signals in asecond direction opposite to the first direction wherein the PR and theSR are interconnected by a plurality of reconfiguration units. Each ofthe reconfiguration units has a PR input, a PR output, a SR input, a SRoutput, a plurality of attachment ports suitable for receiving signalsfrom data terminals or the like, at least one medium access controller(MAC) for receiving and/or generating signals on the PR and switchingunits for selectively wrapping the PR and SR. In the particularembodiment of the method of the present invention the reconfigurationunits detect a failure on the PR and generate a failure frame on the PRwherein the failure frame contains the address of the first MAC whichdetected the failure and generated the failure frame. The MAC alsodetermines if a failure frame is received from the PR and connects thefirst attachment port to the SR input while isolating the SR input fromthe SR output, isolating the PR input from the first attachment port andconnecting the PR input to the SR output if the MAC generating thefailure frame has not received the failure frame from the PR within apreselected time. The reconfiguration unit thereby bypasses the failureby wrapping the PR to the SR at the upstream side of the reconfigurationunit and wrapping the attachment ports and MAC to the downstream ring.

The reconfiguration unit detecting the failure may also have switchingunits for selectively isolating the ring of the reconfiguration unit. Insuch a case, the reconfiguration method further involves isolating thereconfiguration unit associated with the MAC generating the failureframe from the PR and SR of the dual ring communication network if theMAC generating the failure frame has not received the failure frame fromthe PR within a preselected time. The reconfiguration unit is isolatedby isolating the attachment ports and the MAC from the PR input, the PRoutput, the SR input and the SR output while maintaining an internalring between the ports of the reconfiguration unit and the MAC of thereconfiguration unit and while connecting the PR input to the SR outputand the PR output to the SR input. Thus, a ring internal to thereconfiguration unit is formed and the external rings are wrapped toprevent interruption of the other reconfiguration units in the network.

The method also includes testing the isolated reconfiguration unit todetermine if the failure was caused by the isolated reconfiguration unitand reconnecting the isolated reconfiguration unit to the PR and SR whenthe testing determines that the isolated reconfiguration unit did notcause the failure. The isolated reconfiguration unit is reconnected tothe PR and SR by disconnecting the PR output from the SR input andconnecting the first attachment port to the SR input and the MAC outputto the PR output while maintaining the connection between the PR inputand SR output and the isolation from the PR input and SR output.

The method of the present invention may also include waiting apreselected time after determining the downstream reconfiguration unitdid not cause the failure and then reevaluating the PR and SR connectionbetween the upstream reconfiguration unit and the downstreamreconfiguration unit by reconnecting the input of the first attachmentport of the downstream reconfiguration unit to the PR input, the outputof the MAC to the PR output and the SR input to the SR output whiledisconnecting the SR output from the PR input and monitoring the PR todetermine if a failure continues to occur. If the failure continues tooccur the failure is bypassed by disconnecting the downstreamreconfiguration unit PR and SR from the PR input and SR output of thedownstream reconfiguration unit, connecting the PR input to the SRoutput and connecting PR to SR prior to the first port of the downstreamreconfiguration unit.

In an IEEE 802.5 compatible embodiment of the present invention afailure is detected by detecting an IEEE 802.5 Type 2, Type 3 or Type 4beacon frame or detecting an absence of transmissions on the PR. Also,the failure frame generated may be an IEEE 802.5 Type 1 beacon framegenerated on the PR and containing the address of the MAC generating theframe as the source address field of the beacon frame.

One embodiment of the present invention provides a method forreconfiguring a concentrator which detects a failure on a ring of a dualring communication system. The method includes detecting a failure onthe PR and generating a failure frame on the PR wherein the failureframe contains the address of the MAC of the concentrator detecting thefailure. The concentrator then determines if a failure frame is receivedon the PR with the address of the MAC of the concentrator. If the frameis not received within a preselected time the concentrator bypasses thefailure by connecting the first attachment port to the SR input of theconcentrator while isolating the SR output from the SR input and the PRinput from the first attachment port and connecting the SR output to thePR input.

The method of reconfiguring a concentrator detecting a failure may alsoinclude isolating the concentrator generating the failure frame from thePR and SR of the dual ring communication network if the MAC generatingthe failure frame has not received the failure frame from the PR withina preselected time. The isolation is accomplished, by isolating theattachment ports and the MAC from the PR input and output and the SRinput and output while maintaining an internal ring between the ports ofthe concentrator and the MAC of the concentrator and while connectingthe PR input to the SR output and the PR output to the SR input. Theconcentrator may then test the isolated concentrator to determine if thefailure was caused by the isolated concentrator. If the failure was notcaused by the isolated concentrator the isolated concentrator isreconnected to the PR and SR by disconnecting the PR output from the SRinput and connecting the first attachment port to the SR input and theMAC output to the PR output while maintaining the connection between thePR input and SR output and the isolation from the PR input and SRoutput.

An additional aspect of the present invention includes a method forreconfiguring a concentrator situated upstream of the concentratordetecting a failure. The method includes receiving a failure frame onthe PR and wrapping the PR to the SR at the PR output and SR input whenthe failure frame contains a source address corresponding to the MAC ofthe nearest downstream concentrator while connecting the MAC output tothe SR output and isolating the MAC output from the PR output andisolating the SR output from the SR input. The method also includesrepeating the failure frame on the PR when the source address of thefailure frame is not the address of the nearest downstream concentrator.

The concentrator may also have a switching unit for selectivelyisolating the MAC of the concentrator from the PR and the SR andwrapping the input of the MAC to the output of the MAC. The method thenfurther includes isolating from the PR and the SR the MAC of theconcentrator when a failure frame is received which has a source addressof the MAC of the nearest downstream concentrator and wrapping the PRand the SR of the concentrator by connecting the PR to the SR after thelast port and before the MAC of the concentrator and connecting the PRoutput to the SR input. The method also tests the isolated MAC todetermine if the MAC is the cause of the failure and then reconnects theisolated MAC if the testing determines that the MAC is not the cause ofthe failure. The isolated MAC is reconnected by reconnecting the MAC tothe PR and SR while maintaining the isolation from the PR output and theSR input and relocating the wrap of the PR and SR to after the MAC ofthe concentrator.

A particular embodiment of the apparatus of the present inventionprovides a reconfiguration unit for a dual ring communication systemhaving a primary ring input, a primary ring output, a secondary ringinput, a secondary ring output and a plurality of attachment portshaving a ring in connection and a ring out connection suitable forreceiving signals from data terminals and the like. The reconfigurationunit also has a medium access control unit (MAC) having a ring inconnection and a ring out connection which is connected downstream ofthe last active attachment port for receiving, generating and/orrepeating signals. The reconfiguration unit further includes portreconfiguration means connected to the primary ring input, the primaryring output, the secondary ring input, the secondary ring output, theMAC and the plurality of attachment ports for selectively connecting theinput ports, the output ports, the MAC and the attachment ports to forma dual ring communication system between active attachment ports, theMAC and the inputs and outputs in response to control signals. Thereconfiguration unit includes control means connected to the MAC and theport reconfiguration means for providing the control signals to the portreconfiguration means and for receiving and transmitting signals to theMAC. Downstream wrap means responsive to the control means are alsoincluded for selectively disconnecting the SR input from the SR output,disconnecting the PR input from the ring in connection of the firstactive attachment port and for connecting the ring in connection of thefirst active attachment port to the SR input and connecting the PR inputto the SR output.

The reconfiguration unit may also include internal ring connection meansresponsive to the control means for selectively connecting the ring inand ring out connections of the active attachment ports to the ring inand ring out connections of the MAC to form a ring of the activeattachment ports and the MAC internal to the reconfiguration unit. MACisolation means responsive to the control means may also be included inthe reconfiguration unit for selectively isolating the MAC from theattachment ports and the input and output ports and for connecting thering in connection to the ring out connection of the MAC whileconnecting the PR output port to the SR input port and connecting thering out connection of the last active attachment port to the SR outputport.

Additional aspects of the present invention include reconfigurationunits having reconfiguration unit isolation means responsive to thecontrol means for selectively activating the internal ring connectionmeans and for connecting the PR output to the SR input and the PR inputto the SR output. Also included may be upstream wrap means responsive tothe control means for connecting the PR output to the SR input andconnecting the ring out connection of the MAC to the SR output.

In an additional embodiment of the present invention a dual ringcommunication system capable of fault isolation and fault bypass isprovided. The dual ring communication system has at least tworeconfiguration units connected so as to form a ring wherein at leastone of said reconfiguration units is a single medium access controller(MAC) reconfiguration unit. A dual ring communication system is alsoprovided wherein each of the reconfiguration units is a single MACreconfiguration unit.

In view of the above, these aspects of the present invention provide alower cost reconfiguration unit capable of use in a dual ringcommunication system having multiple reconfiguration units. Because thereconfiguration units of the present invention utilize only a singleadapter to the communication ring, the cost of such systems is reduced.Furthermore, because the reconfiguration units perform the faultisolation and bypass by wrapping the PR to the SR at the inputs oroutputs of the unit the integrity of the ring external to the units maybe maintained with only minimal disruption.

Also provided is a method of fault isolation and bypass capable of usein multiple reconfiguration unit networks. The present invention alsoprovide a reconfiguration units and methods of fault isolation andbypass which are compatible with both existing hardware and with theIEEE 802.5 standard. The methods and apparatus of the present inventionutilize existing commands and frame formats to detect and isolate thefaults. Finally, the methods of the present invention provide a methodof fault isolation and bypass allow for use with reconfiguration unitshaving only one MAC or adapter to the dual ring communication systemthus making possible the advantages of the reconfiguration units of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a dual ring communication system comprisedof reconfiguration units of the present invention.

FIG. 2A is a block diagram of a reconfiguration unit according to thepresent invention in the MAC isolation state.

FIG. 2B is a block diagram of a reconfiguration unit according to thepresent invention in the Internal Isolation state.

FIG. 2C is a block diagram of a reconfiguration unit according to thepresent invention in the Upstream Wrap state.

FIG. 2D is a block diagram of a reconfiguration unit according to thepresent invention in the Downstream Wrap state.

FIG. 3 is a block diagram of one embodiment of a reconfiguration unitaccording to the present invention.

FIG. 4A, FIG. 4B, and FIG. 4C is a flow chart of one embodiment of theoperations performed by a reconfiguration unit according to the methodof fault isolation and bypass of the present invention.

FIG. 5A and FIG. 5B is a flow chart of one embodiment of the operationsperformed by a reconfiguration unit as a method of determining theaddress of the nearest downstream reconfiguration unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Describedbelow is one particular embodiment of the present invention as part ofan IEEE 802.5 Token Ring Network. As will be appreciated by one ofordinary skill in the art, the principles of the present invention maybe applied to other forms of dual ring communication systems.

FIG. 1 illustrates a dual ring communication system having a primarydata path 5 or primary ring and a secondary data path 6 or secondaryring. As seen in FIG. 1, data is transmitted in a first direction on theprimary data path 5 and in a second, opposite direction on the secondarydata path 6. Data is transmitted sequentially along the active data pathand passes through each of the attachments interconnected to the datapath. Included in the dual ring communication system illustrated in FIG.1 are reconfiguration units 10, 20, and 30. These reconfiguration units10, 20, and 30 may be concentrators for providing attachment ports 13,23, and 33 to the dual ring communication system for interconnection ofworkstations and the like to the primary or secondary data paths. Theseworkstations are interconnected to form a ring of sequentially connecteddevices. Thus, each attachment port will have a ring in and a ring outwhich are selectively connected to other active attachment ports to formthe data ring. Methods of such selective attachment are described incommonly assigned U.S. Pat. No. 5,132,962 the disclosure of which isincorporated herein as if set forth fully. For illustrative purposes aworkstation 24 capable of receiving, transmitting and/or optionallygenerating signal on the active data path is shown as attached toreconfiguration unit 20 at attachment ports 23. The devices connected tothe attachment ports should at a minimum be capable of propagating thesignals received at the ring in of the device to the ring out of thedevice. While a workstation has been described as attached to theattachment ports, as will be understood by one of skill in this art,other attachments or devices may be interconnected to the dual ringcommunication system through the attachment ports.

FIG. 1 further illustrates the reconfiguration units 10, 20, and 30which have inputs 11, 21, and 31 and outputs 16, 26, and 36 which areinterconnected to form the primary data path 5. Thus in operation,information is transmitted from a device or workstation connected toreconfiguration unit 20 would be transmitted on the cable connected toprimary ring output 26 and received by reconfiguration unit 30 atprimary ring input 31. The information would then propagate throughreconfiguration unit 30 and be transmitted to the cable connected toprimary ring output 36. The information would then be received atprimary ring input 11 of reconfiguration unit 10 and propagate throughreconfiguration unit 10. Reconfiguration unit 10 would then transmit theinformation on the cable connected to primary ring output 16 and theinformation would be received at primary ring input 21 ofreconfiguration unit 20.

The reconfiguration units 10, 20, and 30 also have outputs 12, 22, and32 and inputs 17, 27, and 37 which are interconnected to form thesecondary data path 6. The secondary data path typically operates onlyin the case of a fault. However, the secondary ring output 12 ofreconfiguration unit 10 is connected to the secondary ring input 37 ofreconfiguration unit 30, the secondary ring output 32 of reconfigurationunit 30 is connected to the secondary ring input 27 of reconfigurationunit 20 and the secondary ring output 22 of reconfiguration unit 20 isconnected to the secondary ring input 17 of reconfiguration unit 10.

Finally, illustrated in FIG. 1 is an adapter or medium access controlunit (MAC) 15, 25, and 35 which is part of each of the reconfigurationunits. The function of MAC 25 is to provide an interface to the primarydata path. The MAC in each reconfiguration unit is connected to theprimary data path 5 downstream of the attachment ports. As used herein,downstream refers to sequentially following in the direction of datatraffic on the active path and upstream refers to sequentially precedingin the direction opposite to data traffic on the active data path. Forexample, in FIG. 1, reconfiguration unit 20 is downstream ofreconfiguration unit 10 and upstream of reconfiguration unit 30 whendata is transmitted on the primary data path 5. The MAC may be fullycompatible with the IEEE 802.5 Medium Access Control layer or may onlyimplement the error recovery subset of the MAC layer of the IEEEstandard. Furthermore, for purposes of the present invention, only thosefault detection and recovery functions described below need beimplemented in the MAC.

While the above dual ring communication network has been described withrespect to three single MAC reconfiguration units, any number ofreconfiguration units may be interconnected to form the dual ringcommunication system. Furthermore, the reconfiguration units of thepresent invention may be interconnected with other reconfiguration unitssuch as those described in U.S. Pat. No. 5,132,962 having three adaptersconnected to the various rings of the dual ring communication systemswhile still maintaining the capabilities and benefits of fault isolationand bypass of the present invention and of the multiple adapterreconfiguration units.

Particular aspects of the methods of the present invention will now bedescribed with reference to FIG. 1 and FIG. 2.

Failures along the ring may be divided into four categories based uponthe location of the failure. First, the failure may occur in theinterconnection between reconfiguration units. Second, the failure mayoccur internal to the MAC associated with a particular reconfigurationunit. Third, the failure may occur internal to the reconfiguration unitbut external to the MAC of the reconfiguration unit. Finally, thefailure may occur within the individual device or the connection betweenthe individual device and the particular attachment port of thereconfiguration unit. Each of these failure locations may be isolatedand bypassed utilizing the apparatus and methods of the presentinvention.

When a failure occurs, the failure is detected either by the nearestattached device or the nearest MAC downstream of the failure. Thus, ifthe failure occurs between reconfiguration units, within the MAC orwithin an attachment to a reconfiguration unit, then the failure will bedetected by the first MAC of a reconfiguration unit downstream of thefailure. For purposes of illustration, the isolation of a failure in theprimary ring 5 between reconfiguration unit 10 and reconfiguration unit20 will be described.

After the error on primary ring 5 occurs, workstation 24 notifies MAC 25of the failure. When the failure is detected, the MAC 25 generates afailure frame on the primary ring 5 which contains the address of MAC 25which detected the failure and generated the failure frame. MAC 25 thendetermines if the failure frame it generated is received from theprimary ring 5. If the failure frame is not received from the primaryring 5 within a preselected time, reconfiguration unit 20 enters thedownstream wrap state illustrated in FIG. 2D. References to thedownstream wrap state refer to the wrap state of the reconfigurationunit having a physical location downstream of the failure location. Inthe downstream wrap state the first active attachment port ofreconfiguration unit 20 is connected to the secondary ring input 27while the secondary ring input 27 is isolated from the secondary ringoutput 22. Also in the downstream wrap state, the primary ring input 21is isolated from the first attachment port and the primary ring input 21is connected to the secondary ring output 22.

Optionally, before entering the downstream wrap state, reconfigurationunit 20 could enter the internal isolation state depicted in FIG. 2B. Inthe internal isolation state, the reconfiguration unit associated withthe MAC generating the failure frame is isolated from the primary andthe secondary rings. In the present case, reconfiguration unit 20 isisolated by isolating the attachment ports 23 and the MAC 25 from theprimary ring input 21 and output 26 and the secondary ring input 27 andoutput 22 while maintaining an internal ring between the attachmentports 23 and MAC 2B and while connecting the primary ring input 21 tothe secondary ring output 22 and the primary ring output 26 to thesecondary ring input 27. Reconfiguration unit 20 then tests the internalring to determine if the failure was caused internal to reconfigurationunit 20. If it is determined that the error was not internal toreconfiguration unit 20, then reconfiguration unit 20 would enter thedownstream wrap state described above and depicted in FIG. 2D. Thedownstream wrap state is entered from the internal isolation state bydisconnecting the primary ring output 26 from the secondary ring input27 and connecting the first attachment port of reconfiguration unit 20to the secondary ring input 27 and connecting the output of MAC 25 tothe primary ring output 26 while maintaining the connection between theprimary ring input 21 and secondary ring output 22 and the isolationfrom the primary ring input 21 and secondary ring output 22.

Reconfiguration unit 20 may also optionally wait a preselected timeafter determining that the failure was not internal to reconfigurationunit 20 and then reevaluate the primary ring and secondary ringconnections to the upstream reconfiguration unit 10 and the downstreamreconfiguration unit 30 by reconnecting the input of the firstattachment port of reconfiguration unit 20 to the primary input 21, theoutput of MAC 25 to the primary output 26 and the secondary ring input27 to the secondary ring output 22 while disconnecting the secondaryring output 22 from the primary ring input 21. Reconfiguration unit 20would then monitor the primary ring to determine if a failure continuesto occur. If a failure continues to occur, reconfiguration unit 20 wouldthen reenter the downstream wrap state described above.

With regard to the downstream reconfiguration unit 30, when MAC 35received the failure frame with the address of MAC 25, reconfigurationunit 30 determines that the failure frame did not come from the nearestdownstream reconfiguration unit to reconfiguration unit 30 and sorepeats the failure frame on the primary ring.

When reconfiguration unit 10 receives the failure frame, it determinesthat the failure frame was generated by the nearest downstreamreconfiguration unit as so reconfiguration unit 10 enters upstream wrapstate as depicted in FIG. 2C. References to the upstream wrap staterefer to the wrap state entered by the reconfiguration unit having aphysical location upstream of the failure location. Reconfiguration unit10 enters the upstream wrap state by wrapping the primary ring to thesecondary ring at the primary ring output 16 and the secondary ringinput 17. Reconfiguration unit 10 also wraps the output of MAC 15 to thesecondary ring output 12.

Optionally, reconfiguration unit 10 could enter the MAC isolation statedepicted in FIG. 2A and then evaluate MAC 15 to determine if MAC 15caused the failure. To enter the MAC isolation state, reconfigurationunit 10 isolates the primary ring and the secondary ring from MAC 15 andwraps the primary ring and the secondary ring of reconfiguration unit 10by connecting the primary ring to the secondary ring after the lastattachment port and before MAC 15 and connecting the primary output 16to the secondary input 17. Reconfiguration unit 10 then tests theisolated MAC 15 to determine if MAC 15 caused the failure.Reconfiguration unit 10 then reconnects the isolated MAC 15 if thetesting determines that MAC 15 did not the cause the failure. MAC 15 isreconnected by reentering the upstream wrap state described above.

As with the downstream reconfiguration unit 20, the upstreamreconfiguration unit 10 may optionally retest the ring to determine ifthe failure has been repaired. After a preselected time expires and ifMAC 15 was not the cause of the failure, reconfiguration unit 10reevaluates the primary ring and the secondary to ring connectionbetween reconfiguration unit 10 and reconfiguration unit 20 by enteringthe normal connection state depicted in FIG. 1. The normal connectionstate is entered by reconnecting the primary ring internal toreconfiguration unit 10 to primary ring output 16 and the secondary ringinternal to reconfiguration unit 10 to the secondary ring input 17 andthen monitoring the primary ring to determine if a failure continues. Ifthe failure continues to occur, reconfiguration unit 10 reenters theupstream wrap state as described above.

One particular embodiment of the apparatus of the present invention isshown in FIG. 3. FIG. 3 is a block diagram of a single MACreconfiguration unit according to the present invention. For purposes ofconsistency, FIG. 3 has been labeled consistent with reconfigurationunit 20 of FIG. 1 with like elements having the same numericidentification. As seen in FIG. 3, reconfiguration unit 20 is connectedto the primary ring 5 through the primary ring input 21 and the primaryring output 26. Reconfiguration unit 20 is connected to the secondaryring 6 through the secondary ring input 27 and the secondary ring output22. Reconfiguration unit 20 also has a plurality of attachment ports 23each having a ring out connection 41 and a ring in connection 42suitable for receiving signals from data terminals and the like. Mediumaccess control unit (MAC) 25 is connected within reconfiguration unit 20downstream of the last of the plurality of attachment ports 23 and has aring in connection 28 and a ring out connection 29 which connect MAC 25to the primary ring. MAC 25 is capable of receiving signals at the ringin connection 28, generating signals at the ring out connection 29,and/or repeating signals received at the ring in connection 28 on thering out connection 29.

Reconfiguration unit 20 also contains port reconfiguration means 40connected to the primary ring 5, MAC 25 and the plurality of attachmentports 23. The port reconfiguration means selectively connects the activeinput ports (i.e. input ports with functional devices attached) to theprimary ring so as to form a path for the primary ring through theactive devices. The port reconfiguration means may also selectivelyisolate particular ports which have failing attachments to therebybypass the failing attachment. One implementation of a portreconfiguration means is described in U.S. Pat. No. 5,132,962 at column3 line 43 through column 4 line 43, the disclosure of which isincorporated herein by reference. However, in the present invention asonly one MAC or adapter is present, the reconfiguration switch of the'962 patent would receive the configuration information from themicroprocessor 41 which acts as a control means for the reconfigurationunit 20. Furthermore, as will be appreciated by one of skill in thisart, the control means of reconfiguration unit 20 may include amicroprocessor, an ASIC, or other programmable devices which may carryout the described functions described below. The control means ofreconfiguration unit 20 may also, optionally, be incorporated in MAC 25.

In addition to the above elements for creating and maintaining a ringpath through active attachments, the reconfiguration unit 20 also haselements for selectively wrapping and bypassing elements of thereconfiguration unit for fault isolation and fault recovery. On theupstream side of the port reconfiguration means 40 the isolation andbypass elements include isolation switches 43 and 46 and wrap switches44 and 45. Between the port reconfiguration means 40 and MAC 25 theisolation and bypass elements include isolation switches 47 and 50 andwrap switches 48 and 49. Downstream of MAC 25 the isolation and bypasselements include isolation switches 51 and 54 and wrap switches 52 and53. As will be appreciated by one of skill in this art, isolation andwrap switches may be relays, transistors or other switching deviceswhich can selectively connect one signal path to a second signal path.

As seen in FIG. 3, isolation switch 43, which is responsive to controlsignals from microprocessor 41, selectively disconnects the primary ring5 at the primary ring input 21 from the port reconfiguration means 40and isolation switch 46, which is also responsive to control signalsfrom microprocessor 41, selectively disconnects the secondary ring 6 atthe secondary ring output 22 from the secondary ring input 27. In normaloperation isolation switches 43 and 46 are in the closed or connectedstate where isolation switch 43 connects the primary ring 5 to the portreconfiguration means 40 and isolation switch 46 connects the secondaryring 6 to isolation switch 50.

Reconfiguration unit 20 also includes wrap switch 45 which is responsiveto control signals from the microprocessor 41 and which selectivelyconnects the primary ring 5 between the primary ring input 21 andisolation switch 43 to the secondary ring 6 between the secondary ringoutput 22 and isolation switch 46. Wrap switch 44 is responsive tocontrol signals from the microprocessor 41 and selectively connects theprimary ring entering the port reconfiguration means 40 to the secondaryring between isolation switches 46 and 50. In normal operation wrapswitches 44 and 45 are in the open or disconnected state such that theprimary ring and secondary ring are not connected.

Between the port reconfiguration means 40 and MAC 25 the isolation andbypass elements include isolation switch 47 which is responsive tocontrol signals from microprocessor 41 and selectively disconnects theoutput of the primary ring of port reconfiguration means 40 from thering in 28 of MAC 25. Isolation switch 50 is also responsive to controlsignals from microprocessor 41 and selectively disconnects the secondaryring output 22 from the secondary ring input 27. In normal operation,isolation switches 47 and 50 are in the closed or connecting state. Wrapswitch 48 is responsive to microprocessor 41 and selectively connectsthe primary ring between isolation switch 47 and MAC 25 and thesecondary ring between isolation switch 50 and the connection point ofwrap switch 53. Wrap switch 49 is responsive to microprocessor 41 andselectively connects the primary output of the port reconfigurationmeans 40 to the secondary ring path internal to the reconfigurationunit. In normal operation, wrap switches 48 and 49 are in the open ordisconnected state.

The final isolation and bypass elements are located downstream of MAC 25and include isolation switch 51 which is responsive to microprocessor 41and selectively disconnects the ring out 29 of MAC 25 from the primaryring output 26 of reconfiguration unit 20. Isolation switch 54 is alsoresponsive to microprocessor 41 and disconnects secondary ring input 27from the secondary ring output 22. In normal operation, isolationswitches 51 and 54 are in the closed or connection state. Wrap switch 52is responsive to microprocessor 41 and selectively connects the ring out29 of the MAC 25 to the secondary ring between isolation switch 54 andthe connection of wrap switch 48 to the secondary ring ofreconfiguration unit 20. Wrap switch 53 is responsive to microprocessor41 and selectively connects the primary ring output 26 between isolationswitch 51 and primary ring output 26 to the secondary ring input 27between the secondary ring input 27 and isolation switch 54. In normaloperation wrap switches 52 and 53 are in the open or disconnected state.

Reconfiguration unit 20 has been described with reference to individualswitching units 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53 and 54. Aswill be appreciated by one of skill in this art, each grouping ofswitches may be implemented using a single relay having two normallyopen contacts and two normally closed contacts. For example, switches43, 44, 45, and 46 could be implemented using a single relay withnormally open contacts for switches 44 and 45 and normally closedcontacts for switches 43 and 46. When the relay actuates the normallyopen contacts close and the normally closed contacts open, thus causingthe isolation and wraps desired. While these switches may be implementedusing a single relay, as will be appreciated by one of skill in thisart, other switching devices may be employed such as transistors ormultiple relays.

While reconfiguration unit 20 has been described in detail above, forpurposes of the discussion regarding operation of the reconfigurationunits, reconfiguration units 10 and 30 should be considered as havingcorresponding microprocessor, port reconfiguration means, wrap switchesand isolation switches. Accordingly, these elements of reconfigurationunits 10 and 30 will be referred to herein by the reference numerals ofFIG. 3.

In normal operation, signals are transmitted serially along the primaryring 5 from the workstation or MAC generating the signals to the nextactive device on the primary ring. These devices then generate or repeatthe signals along the primary ring based upon the content of thesignals. However, when a device or connection fails, the primary ring nolonger forms a complete ring and this error is detected by devices alongthe ring. The isolation and bypass of such a failure will now bedescribed with regard to a particular reconfiguration unit 20 asillustrated in the Figures.

When the failure occurs in the interconnection between thereconfiguration units, for example on the primary ring 5 betweenreconfiguration unit 10 and reconfiguration unit 29 in FIG. 1, the firstactive device 24 on reconfiguration unit 20 detects the break incommunications around the ring. Such detection may occur simply by thedevice detecting that no activity occurs along the ring. When such anerror is detected, device 24 enters Beacon Transmit mode and transmits aType 2, Type 3 or Type 4 Beacon frame on the ring depending upon how theerror was detected by the device as defined by the IEEE 802.5 standard.The Beacon frame is repeated by each of the active devices attached toreconfiguration unit 20 until the Beacon frame reaches MAC 25. If noactive devices are attached to reconfiguration unit 20 MAC 25 may detectthe failure. If the error is detected by MAC 25 because no activedevices are attached to reconfiguration unit 20, MAC 25 enters BeaconTransmit mode but, as described below, does not transmit the Type 2,Type 3 or Type 4 Beacon frame on the ring. When MAC 25 receives a Beaconframe or detects a failure the operations as described in the flow chartof FIG. 4A, FIG. 4B and FIG. 4C are carried out by MAC 25 andmicroprocessor 41.

As seen in FIG. 4A at decision block 101, MAC 25 and microprocessor 41determine if the Beacon frame received is a Type 1 (BNR) or a Type 2,Type 3 or Type 4 (BNN) Beacon frame. Because the error occurred betweenthe MAC 15 of reconfiguration unit 10 and reconfiguration unit 20 theBeacon frame will be a BNN frame which is received by MAC 23. As usedherein and in the figures, RBNN will refer to Type 2, Type 3 or Type 4beacon frames received by a MAC of a reconfiguration unit. TBNN shall beused to refer to Type 2, Type 3 or Type 4 Beacon frames which are to betransmitted by a MAC of a reconfiguration unit. Had the error beendetected by MAC 25 then MAC 25 would prepare the appropriate BNN frameas a TBNN frame but would not transmit the frame on the ring. In eithercase, the "yes" branch will be taken and block 102 will be carried outconverting the RBNN or TBNN frame to a Type 1 Beacon frame (BNR) andtransmitting the Type 1 Beacon frame on the ring. Block 103 is thencarried out and Response Timer (T1) is begun. As illustrated in blocks104 and 105, the MAC 25 and the microprocessor 41 then wait to see if aBNR frame is received by MAC 25 within the time set for Response Timer(T1). Time of from about 7.0 to about 7.5 seconds are preferred butother times may be used based upon a particular network configuration.

If a BNR frame is received within the time T1, then, the "yes" path ofblock 104 is taken and reconfiguration unit 20 enters Beacon RepeatMode. The procedures then carried out by reconfiguration unit 20 are thesame as if reconfiguration unit 20 were a reconfiguration unitdownstream of the reconfiguration unit generating the BNR frame.Accordingly, these procedures will be described below with respect toreconfiguration units which receive a BNR frame

If a BNR frame is not received within the time T1, then, as indicated atblock 112, reconfiguration unit 20 enters the internal isolation wrapstate shown in FIG. 2B. Microprocessor 41 activates isolation switches43 and 46 to enter the open or disconnecting state and thereby isolatesreconfiguration unit 20 and attachment ports 23 from primary ring input21 and secondary ring output 22. Microprocessor 41 also activates wrapswitch 45 to the closed or connecting state to wrap the primary ringinput 21 to the secondary ring output 22. Microprocessor 41 alsoisolates the ring out 29 of MAC 25 from the primary ring output 26 byactivating isolation switch 51 to enter the open or disconnecting state.Reconfiguration unit 20 is isolated from secondary ring input 27 byactivation of isolation switch 54 to the open or disconnecting state.Primary ring output 26 is wrapped to secondary ring input 27 byactivation of wrap switch 52 to the closed or connecting state. Tocreate an internal ring between the devices attached to reconfigurationunit 20 and MAC 25, microprocessor 41 activates wrap switches 44 and 53which enter the closed or connecting state and which connect the primaryring input of port reconfiguration means 40 to the internal secondaryring between isolation switches 46 and 50 and which connects the ringout 29 of MAC 25 to the internal secondary ring between isolationswitches 50 and 54.

If no errors are detected on the internal ring formed by reconfigurationunit 20 entering the internal isolation wrap state then the "yes" pathof block 113 is taken. Because no errors occurred during the internalwrap, the error must have occurred upstream of reconfiguration unit 20.Reconfiguration unit 20 then unwraps the ring out side of the unit andenters the downstream unit wrap state indicated in FIG. 2D. Thedownstream wrap state is entered by microprocessor 41 deactivating wrapswitches 52 and 53 and deactivating isolation switches 51 and 54. Thus,the isolated reconfiguration unit 20 is reconnected to the primary andsecondary rings by disconnecting the primary ring output 26 from thesecondary ring input 27, connecting the ring out 29 of MAC 25 to theprimary ring output 26 and connecting the internal secondary ring atisolation switch 54 to the secondary ring input 27. Reconfiguration unit20 also loads and starts a Downstream Retest Timer (T3) as reflected inblock 115 of FIG. 4B. Times of from about 4 to about 5 minutes arepreferred for T3. However, other times may be used based upon aparticular network implementation or user preference.

As seen in blocks 118 and 119 of FIG. 4B, upon expiration of T3reconfiguration unit 20 unwraps the input side of the unit and reentersthe normal connection mode. Normal connection is reestablished bydeactivating wrap switches 44 and 45 and isolation switches 43 and 46 todisconnect the primary ring from the secondary ring and reconnect thesecondary ring internal to the reconfiguration unit to the secondaryring output 22 and the primary ring input of port reconfiguration means40 to primary ring input 21. If errors continue to occur,reconfiguration unit 20 reenters the downstream wrap state shown in FIG.2D and described above. Optionally, reconfiguration unit 20 couldreinitiate the Downstream Retest Timer and retest the ring after thetimer expires or could simply notify the system administrator that theerror was not corrected in time T3 and that manual intervention wasrequired to return the reconfiguration unit to normal connection mode.

If errors are detected on the internal ring formed by reconfigurationunit 20 entering the internal isolation wrap state then the "no" path ofblock 113 is taken. If an error is detected in the internal isolationwrap state then the error was caused by a device attached toreconfiguration unit 20 or is internal to reconfiguration unit 20.Reconfiguration unit 20 then maintains the internal isolation wrap stateand performs internal isolation procedures reflected in block 114 toisolate the fault and bypass the fault if possible. In a preferredembodiment the fault detection and isolation procedure are carried outas described in copending and commonly assigned application entitled"DUAL FAULT RING ISOLATION" and having U.S. application Ser. No.08/386,384, the disclosure of which is incorporated herein by referenceas if set forth fully. However, as will be appreciated by one ofordinary skill in this art, in the present invention the functions ofthe "S" adapter of the copending application would be carried out by MAC25. If the internal isolation procedure successfully bypasses the sourceof the fault then reconfiguration unit 20 may return to normalconnection mode as described above.

When any devices attached to reconfiguration unit 30 receive the BNRframe transmitted by MAC 25, these device enter Beacon Repeat mode andrepeat the beacon frame on the ring. When MAC 35 receives the beaconframe it first tests to determine if the frame is a RBNN or TBNN frame.Because the frame is a BNR frame, the "no" branch of block 101 is takenand the test of block 106 is performed to see if the frame is a BNRframe. If the frame is a BNR frame then reconfiguration unit 30 entersBeacon Repeat Mode as reflected in block 107. In Beacon Repeat Mode, MAC35 repeats the BNR frame on the primary ring. Reconfiguration unit 30then evaluates the source address field of the BNR frame to determine ifthe reconfiguration unit generating the BNR frame is the nearestdownstream reconfiguration unit to reconfiguration unit 30.

Each of the reconfiguration units keeps track of the number ofconsecutive BNR frames received from the nearest downstreamreconfiguration unit in a counter referred to herein as the BeaconCounter. As seen in blocks 108 and 110, the Beacon Counter is initiallyzero and is incremented each time a BNR frame is received with a sourceaddress of the nearest downstream reconfiguration unit to the unitreceiving the BNR frame. If an intervening BNR frame with a differentsource address is received then the Beacon Counter is reset to zero. Toallow the beacon condition to settle to the correct fault domain, asreflected in block 111, no fault isolation or recovery action other thanrepeating the BNR frame is carried out until 8 consecutive BNR framesare received with the source address of the nearest downstreamreconfiguration unit to the unit receiving the BNR frame. While thepresent example has set 8 as the value for the Beacon Counter, as willbe appreciated by one of skill in this art, other values may be used.

Because reconfiguration unit 20 is not the nearest downstreamreconfiguration unit to reconfiguration unit 30 the "no" path of block108 is taken and reconfiguration unit 30 resets its Beacon Counter tozero as reflected in block 109. No further action is taken byreconfiguration unit 30.

As with the devices attached to reconfiguration unit 30, when thedevices attached to reconfiguration unit 10 receive the BNR frame thedevices repeat the frame on the primary ring. When reconfiguration unit10 receives the BNR frame it carries out the testing described above.Because reconfiguration unit 20 which generated the BNR frame is thenearest downstream reconfiguration unit to reconfiguration unit 10,reconfiguration unit 10 will eventually receive 8 consecutive BNR frameswith the source address of reconfiguration unit 20 and the "yes" branchof block ill will be carried out by reconfiguration unit 10.

After receiving the 8 consecutive BNR frames reconfiguration unit 10carries out the procedure of block 120 and enters the MAC isolationstate depicted in FIG. 2A. The MAC isolation state is entered by themicroprocessor 41 of reconfiguration unit 10 activating isolationswitches 47, 51, 54 and 50 of reconfiguration unit 10 to enter the openor disconnecting state and activating wrap switches 48, 49, 52, and 53of reconfiguration unit 10 to enter the closed or connecting state. Theactivation of the above wrap and isolation switches wraps the primaryoutput of the port reconfiguration means 40 of reconfiguration unit 10to the secondary ring internal to the reconfiguration unit. A ring isalso formed from the ring in 28 to the ring out 29 of MAC 15 ofreconfiguration unit 10. Finally, the primary output 16 is wrapped tothe secondary input 17 of reconfiguration unit 10.

If an error is detected by the wrapped MAC 15 of reconfiguration unit 10then reconfiguration unit 10 takes the "no" path of block 121 andcarries out the internal error procedures reflected by block 127. Withthe output of MAC 15 wrapped to the input of MAC 15, microprocessor 41checks the status information of MAC 15 of reconfiguration unit 10. Ifan error is detected in the status information, reconfiguration unit 10enters the internal isolation state and reports the error to the user.

If no errors are detected by the wrapped MAC 15 of reconfiguration unit10 then reconfiguration unit 10 takes the "yes" path of block 121 andenters the upstream wrap state shown in FIG. 2C. Reconfiguration unit 10also starts art Upstream Retest Timer (T2) reflected by block 122.Values of T2 from about 4 to about 5 minutes are preferred; however, aswill be appreciated by one of skill in this art, other values may beused. Upstream wrap state is entered by deactivating isolation switches47, and 50 of reconfiguration unit 10 and deactivation of wrap switches48 and 49 of reconfiguration unit 10. Therefore, entering the upstreamwrap state connects the primary ring output 16 to the secondary ringinput 17 while maintaining the isolation of the ring out 29 of MAC 15from the primary ring output 26 and maintains the isolation of theinternal secondary ring of reconfiguration unit 10. Entering theupstream wrap state also results in a wrap from the ring out 29 of MAC15 to the internal secondary ring of reconfiguration unit 10.

As seen in blocks 123 and 124 of FIG. 4C upon expiration of T2reconfiguration unit 10 unwraps the output side of the unit and reentersthe normal connection mode. Normal connection is reestablished bydeactivating wrap switches 52 and 53 and isolation switches 51 and 54 todisconnect the primary ring from the secondary ring and reconnect theinternal secondary ring of reconfiguration unit 10 to the secondary ringinput 17 and the primary ring input of port reconfiguration means 40 ofreconfiguration unit 10 to primary ring output 16. If errors continue tooccur, reconfiguration unit 10 reenters the upstream wrap state shown inFIG. 2C and described above. Optionally, reconfiguration unit couldreinitiate the Upstream Retest Timer (T2) and retest the ring after thetimer expires or could simply notify the system administrator that theerror was not corrected in time T2 and that manual intervention wasrequired to return the reconfiguration unit to normal connection mode.

As discussed above, to fully implement the error recovery and bypass ofa single MAC reconfiguration unit each of the single MAC reconfigurationunits in the ring should know if a failure frame is generated by thenearest downstream reconfiguration unit to the reconfiguration unitreceiving the failure frame. Furthermore, any method for determining thenearest downstream reconfiguration unit is preferably compatible withexisting reconfiguration units so as to maintain fault isolation andbypass capabilities when single MAC reconfiguration units areincorporated in networks having other types of reconfiguration units.The method described below provides such capabilities.

To determine the address of the nearest downstream reconfiguration unitin a ring network such as that shown in FIG. 1, the reconfigurationunits first arbitrate to determine which reconfiguration unit willassume the Reconfiguration Unit Monitor function on the ring. One methodof determining which station will become the Reconfiguration UnitMonitor would be for the first active reconfiguration unit on the ringcapable of carrying out the monitor functions to become theReconfiguration Unit Monitor. To determine the first activereconfiguration unit capable of carrying out the monitor functions thereconfiguration units wait to see if a predefined frame is received onthe ring within a predetermined time (TM). This time may be either frompower on of the reconfiguration unit or may be from the last receipt ofthe predefined frame. If the predefined frame is received within the TMtime, then another of the reconfiguration units has assumed the role ofReconfiguration Unit Monitor. If the predefined frame is not receivedwithin the TM time then the reconfiguration unit for which the TM timerhas expired generates the predefined frame on the ring and becomes theactive Monitor. Because the reconfiguration units are typically poweredon at different times, the TM timers would expire at different times andthus avoid contention. Also, because the reconfiguration units receivethe predefined frames sequentially, these frames are received atdifferent times for each reconfiguration unit. Accordingly, the TMtimers in the reconfiguration units will be restarted at different timesand contention will again be avoided. Thus, it is preferred that thevalue of TM be set to the same value for each of the reconfigurationunits capable of assuming the status of Reconfiguration Unit Monitor.

Once a Reconfiguration Unit Monitor is established, the reconfigurationunits which are not performing the Monitor function wait for receipt ofa Reconfiguration Unit Map from the Reconfiguration Unit Monitor. TheReconfiguration Unit Map contains the address of each of thereconfiguration units in sequential order around the ring. When theReconfiguration Unit Map is received by the reconfiguration units, thereconfiguration units extract from the map the address of the nearestdownstream reconfiguration unit by locating their own address in thering map and extracting the next address in the map.

To construct the ring map, the Reconfiguration Unit Monitor generates aframe on the ring which requests the address of each reconfigurationunit. The Reconfiguration Unit Monitor then stores the addressesreceived from the other reconfiguration units on the ring and, when allthe other addresses are stored adds its own address to the table ofreconfiguration unit addresses. The Reconfiguration Unit Monitor alsoobtains the address of each attachment to the ring in ring sequenceorder from the neighbor notification process which occurs on the ring.The Reconfiguration Unit Monitor then assembles the ring sequence map ofreconfiguration units by placing the reconfiguration units in thesequence indicated from the neighbor notification process performed onthe ring. The Reconfiguration Unit Monitor then compares this newReconfiguration Unit Map to the previous Reconfiguration Unit Map if oneexisted and, if none existed or if changes are detected, transmits theReconfiguration Unit Map to the other reconfiguration units on the ring.The Reconfiguration Unit Monitor may periodically repeat the aboveprocedure to accommodate any changes to the ring and provide accurate,timely, information to the remaining reconfiguration units.

A particular implementation of the above method on an IEEE 802.5 TokenRing network will now be described with reference to FIG. 1, FIG. 5A andFIG. 5B.

When any of reconfiguration units 10, 20 or 30 is powered on it executesblock 201 of FIG. 5A and loads and starts the timer with the TMpredetermined time value. Each of the reconfiguration units then waitfor the duration of its TM timer to receive a Request Ring StationAttachment frame (RRSA) from the ring as reflected in blocks 202 and206. TM values of from about 14 to about 17 seconds are preferred;however, as will be appreciated by one of skill in this art, othervalues may be used. Receipt of a RRSA frame indicates an activeReconfiguration Unit Monitor. Because each of the reconfiguration unitswas first powered on none of the units would receive the RRSA framewithin the time TM. The reconfiguration units would remain in the202/206 loop until one of the reconfiguration units TM timer expired andthe "yes" branch of the 206 decision block is taken.

For purposes of illustration, it is assumed that reconfiguration unit 20is the first reconfiguration unit to have its TM timer expire. Thus,reconfiguration unit 20 assumes the function of Reconfiguration UnitMonitor. Reconfiguration unit 20 then executes block 207 and generates aRRSA frame on the ring. Reconfiguration unit 20 then loads and startsits Response Timer (TRESP) as reflected in block. 208. Reconfigurationunit 20 then waits for the other reconfiguration units to respond withtheir station addresses. As seen in blocks 209, 210, and 211,reconfiguration unit 20 waits the time TRESP for responses from the RRSAframe and if received saves the address information from the responseframe. TRESP values of about 7 seconds are preferred; however, as willbe appreciated by one of ordinary skill in this art, other values may beused. The address information received in response to the RRSA frame maybe stored by reconfiguration unit 20 in a Response Table. In the presentexample, reconfiguration unit 20 would receive and store addressinformation from reconfiguration unit 30 and reconfiguration unit 10,however, the address information stored in the Response Table would notnecessarily be in ring sequence.

After the Response Timer expires the "yes" branch of decision block 211is taken and, as reflected in decision block 212, reconfiguration unit20 waits for notification on the ring that an attachment has become theActive Monitor. This may be accomplished by receipt of the ActiveMonitor Present frame on the ring.

After receipt of the Active Monitor Present Frame, reconfiguration unit20 checks to see if there were any responses to the RRSA frame. If therewere no responses then there are no other active reconfiguration unitsin the ring and the "no" branch of block 213 is taken. Reconfigurationunit 20 then returns to the Reconfiguration Unit Monitor arbitrationprocess described above. This process periodically repeats to determineif another reconfiguration unit was placed in the ring.

If at least one response to the RRSA frame was received, thenreconfiguration unit 20 takes the "yes" branch from block 213. In thepresent example, reconfiguration unit 20 would take the "yes" branch andadd its own address to the Response Table as indicated in block 214.

To place the address information in the Response Table in ring sequenceorder, reconfiguration unit 20 makes use of a ring map which containsthe address of each active device attached to the ring, includingreconfiguration units, in ring sequence order. This ring map may begenerated during a neighbor notification process which is initiated bythe Active Monitor on the ring. By monitoring the Neighbor Notificationprocess reconfiguration unit 20 can construct the ring map of addressesin ring sequence order. While the ring map contains address informationin ring sequence order it does not identify the type of device at eachaddress. Therefore, reconfiguration unit 20 carries out the stepsreflected in blocks 215, 216, 217, and 218 to identify the location ofaddresses of reconfiguration units in the ring map.

In building a Reconfiguration Unit Map, reconfiguration unit 20 firstextracts an address from the Response Table (215) and finds that addressin the ring map (216). The sequence number of the extracted address isthen saved in a Reconfiguration Unit Map (217). These steps are repeateduntil the sequence numbers of all of the reconfiguration unit addresseshave been placed in the Reconfiguration Unit Map and the "no" branch ofblock 218 is taken. As reflected in block 219 the sequence numbers inthe Reconfiguration Unit Map are then sorted into ring sequence.Reconfiguration unit 20 then compares this newly generatedReconfiguration Unit Map to the previous Reconfiguration Unit Map storedby Reconfiguration Unit 20, if one exists, and if there are no changestakes the "yes" branch of block 220 and starts the Reconfiguration UnitMonitor process again.

If there are changes in the Reconfiguration Unit Map or no previousReconfiguration Unit Map existed, then reconfiguration unit 20 takes the"no" branch of block 220 and extracts and stores the nearest downstreamreconfiguration unit address from the Reconfiguration Unit Map asreflected in block 221. In the present example, reconfiguration unit 20would extract and store the address of reconfiguration unit 30.Reconfiguration unit 20 then overwrites the stored Reconfiguration UnitMap with the new Reconfiguration Unit Map (222) and builds a LogicalLink Control (LLC) frame where the data field of the LLC frame containsthe new Reconfiguration Unit Map (223). This LLC frame is then generatedon the ring and reconfiguration unit 20 starts the Reconfiguration UnitMonitor process again.

For the reconfiguration units which do not become the ReconfigurationUnit Monitor, such as reconfiguration units 30 and 10 in the presentexample, an RRSA frame is received from the Reconfiguration Unit Monitorbefore expiration of the TM timer. Therefore, the "yes" branch ofdecision block 202 is taken. Upon receipt of the RRSA frame,reconfiguration units 10 and 30 respond with the Report Ring StationAttachment frame which contains the source address of the unitgenerating the frame and information about the product in which the ringstation resides. Thus, from the Report Ring Station Attachment frame theReconfiguration Unit Monitor can determine the address of each of theother reconfiguration units on the ring. Also upon receipt of the RRSAframe, reconfiguration units 10 and 30 reset the TM timer as seen inblock 203. After responding to the RRSA frame, reconfiguration units 10and 30 then wait for the LLC frame containing the Reconfiguration UnitMap (204). Once the LLC frame is received, each reconfiguration unitextracts the address of the nearest downstream reconfiguration unit fromthe Reconfiguration Unit Map (205). Reconfiguration units 10 and 30 thenbegin the Reconfiguration Unit Monitor process again, waiting foranother RRSA frame.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, these terms are used in a generic and descriptive sense onlyand not for purposes of limitation, the scope of the invention being setforth in the following claims.

That which is claimed:
 1. A method for reconfiguring a dual ringcommunication system to isolate and bypass failed components orconnections in which the dual ring communication system has a primaryring (PR) for carrying signals in a first direction and a secondary ring(SR) for carrying signals in a second direction opposite to said firstdirection wherein the PR and the SR are interconnected by a plurality ofreconfiguration units, each of said reconfiguration units having a PRinput, a PR output, a SR input and a SR output, a plurality ofattachment ports suitable for receiving signals from data terminals orthe like, at least one medium access controller (MAC) for receiving,generating and/or repeating signals on the PR and switching units forselectively wrapping the PR and SR, the reconfiguration methodcomprising the steps of:detecting a failure on the PR; generating afailure frame on the PR wherein the failure frame contains the addressof the first MAC which detected the failure and generated the failureframe; determining if the failure frame is received from the PR; andconnecting a first attachment port to the SR input while isolating theSR input from the SR output, isolating the PR input from the firstattachment port and connecting the PR input to the SR output if the MACgeneration the failure frame has not received the failure frame from thePR within a preselected time to thereby bypass the failure.
 2. Themethod of claim 1 wherein the reconfiguration unit detecting the failurehas switching units for selectively isolating the ring of thereconfiguration unit and wherein said connecting step further comprisesthe steps of:isolating the reconfiguration unit associated with the MACgenerating the failure frame from the PR and SR of the dual ringcommunication network if the MAC generating the failure frame has notreceived the failure frame from the PR within a preselected time whereinthe reconfiguration unit is isolated by isolating the attachment portsand the MAC from the PR input, the PR output, the SR input and the SRoutput while maintaining an internal ring between the ports of thereconfiguration unit and the MAC of the reconfiguration unit whileconnecting the PR input to the SR output and the PR output to the SRinput; testing the isolated reconfiguration unit to determine if thefailure was caused by the isolated reconfiguration unit; reconnectingthe isolated reconfiguration unit to the PR and SR when said testingdetermines that the isolated reconfiguration unit did not cause thefailure by disconnecting the PR output from the SR input andreconnecting the first attachment port to the SR input and the MACoutput to the PR output while maintaining the connection between the PRinput and SR output and the isolation from the PR input and SR output.3. The method of claim 1 further comprising:wrapping the PR to the SR atthe PR output and SR input of the nearest upstream reconfiguration unitand wrapping the attachment ports to the SR output of the upstreamconfiguration unit.
 4. The method of claim 3 wherein the MAC of theupstream reconfiguration unit can repeat signals on the PR and whereinsaid wrapping step further comprises the steps of:wrapping the PR to theSR at the PR output and SR input of the upstream reconfiguration unitwhich receives the failure frame containing a source address of the MACof the nearest downstream reconfiguration unit to the reconfigurationunit receiving the failure frame while connecting the MAC output to theSR output and isolating the MAC output from the PR output and isolatingthe SR output from the SR input; and repeating the failure frame on thePR when the source address of the failure frame is not the address ofthe nearest downstream reconfiguration unit of the MAC receiving thefailure frame.
 5. The method of claim 4 further comprising the stepsof:waiting a preselected time after determining the downstreamreconfiguration unit did not cause the failure; and then reevaluatingthe PR and SR connection between the upstream reconfiguration unit andthe downstream reconfiguration unit by reconnecting the input of thefirst attachment port of the downstream reconfiguration unit to the PRinput, the output of the MAC to the PR output and the SR input to the SRoutput while disconnecting the SR output from the PR input andmonitoring the PR to determine if a failure continues to occur; anddisconnecting the downstream reconfiguration unit PR and SR from the PRinput and SR output of the downstream reconfiguration unit, connectingthe PR input to the SR output and connecting PR to SR prior to the firstattachment port of the downstream reconfiguration unit if the failurecontinues to occur.
 6. The method of claim 3 wherein the upstreamreconfiguration unit has a switching unit for selectively isolating theMAC of the reconfiguration unit from the PR and the SR and wrapping theinput of the MAC to the output of the MAC and wherein said methodfurther comprises the steps of:isolating from the PR and the SR the MACof the upstream reconfiguration unit receiving the failure frame when afailure frame is received which has a source address of the MAC of thenearest downstream reconfiguration unit; wrapping the PR and the SR ofthe upstream reconfiguration unit by connecting the PR to the SR afterthe last port and before the MAC of the upstream reconfiguration unitand connecting the PR output to the SR input of the upstreamreconfiguration unit; testing the isolated MAC to determine if the MACis the cause of the failure; then reconnecting the isolated MAC if thetesting determines that the MAC is not the cause of the failure byreconnecting the MAC to the PR and SR while maintaining the isolationfrom the PR output and the SR input and relocating the wrap of the PRand SR to after the MAC of the upstream reconfiguration unit.
 7. Themethod of claim 6 further comprising the steps of:reevaluating the PRand SR connection between the upstream reconfiguration unit and thedownstream reconfiguration unit when the MAC of the upstreamreconfiguration unit was determined to not be the cause of the failureand after a preselected time by reconnecting the upstreamreconfiguration unit PR input to the PR output and the SR output to theSR input and monitoring the PR to determine if a failure continues tooccur; and disconnecting the upstream reconfiguration unit PR and SRfrom the PR output and SR input of the upstream reconfiguration unit,connecting the PR output to the SR input and connecting PR to SR of theupstream reconfiguration unit if the failure continues to occur.
 8. Themethod of claim 1 wherein said detecting step comprises detecting anIEEE 802.5 Type 2, Type 3 or Type 4 beacon frame or detecting an absenceof transmissions on the PR.
 9. The method of claim 8 wherein saidgenerating step comprises generating an IEEE 802.5 Type 1 beacon frameon the PR containing the address of the MAC generating the frame as thesource address field of the beacon frame.
 10. A method for reconfiguringa concentrator for a dual ring communication system to isolate andbypass failed components or connections in which the dual ringcommunication system has a primary ring (PR) carrying signals in a firstdirection and a secondary ring (SR) for carrying signals in a seconddirection opposite to said first direction and wherein the concentratorhas a PR input, a PR output, a SR input and a SR output, a plurality ofattachment ports suitable for receiving signals from data terminals orthe like, at least one medium access controller (MAC) for receiving,generating and/or repeating signals on the PR and switching units forselectively wrapping the PR and SR, the steps comprising:detecting afailure on the PR; generating a failure frame on the PR wherein thefailure frame contains the address of the MAC of the concentrator;determining if the failure frame is received on the PR with the addressof the MAC of the concentrator; bypassing the failure when the MACgenerating the failure frame has not received the failure frame from thePR within a preselected time by connecting a first attachment port tothe SR input of the concentrator while isolating the SR output from theSR input and the PR input from the first attachment port and connectingthe SR output to the PR input.
 11. The method of claim 10 wherein theconcentrator has switching units for selectively isolating the ring ofthe concentrator and wherein said bypassing step further comprises thesteps of:isolating the concentrator generating the failure frame fromthe PR and SR of the dual ring communication network if the MACgenerating the failure frame has not received the failure frame from thePR within a preselected time wherein the concentrator is isolated byisolating the attachment ports and the MAC from the PR input and outputand the SR input and output while maintaining an internal ring betweenthe ports of the concentrator and the MAC of the concentrator whileconnecting the PR input to the SR output and the PR output to the SRinput; testing the isolated concentrator to determine if the failure wascaused by the isolated concentrator; reconnecting the isolatedconcentrator to the PR and SR when said testing determines that theisolated concentrator did not cause the failure by disconnecting the PRoutput from the SR input and reconnecting the first attachment port tothe SR input and the MAC output to the PR output while maintaining theconnection between the PR input and SR output and the isolation from thePR input and SR output.
 12. The method of claim 11 further comprisingthe steps of:waiting a preselected time after determining theconcentrator did not cause the failure; and then reevaluating the PR andSR by reconnecting the input of the first attachment port of theconcentrator to the PR input, the output of the MAC to the PR output andthe SR input to the SR output while disconnecting the SR output from thePR input and monitoring the PR to determine if a failure continues tooccur; and disconnecting the concentrator PR and SR from the PR inputand SR output of the concentrator, connecting the PR input to the SRoutput and connecting PR to SR prior to the first attachment port of theconcentrator if the failure continues to occur.
 13. The method of claim10 wherein said detecting step comprises detecting an IEEE 802.5 Type 2,Type 3 or Type 4 beacon frame or detecting an absence of transmissionson the PR.
 14. The method of claim 13 wherein said generating stepcomprises generating an IEEE 802.5 Type 1 beacon frame on the PRcontaining the address of the MAC generating the frame as the sourceaddress field of the beacon frame.
 15. A method for reconfiguring aconcentrator for a dual ring communication system to isolate and bypassfailed components or connections in which the dual ring communicationsystem has a primary ring (PR) carrying signals in a first direction anda secondary ring (SR) for carrying signals in a second directionopposite to said first direction and wherein the concentrator has a PRinput, a PR output, a SR input and a SR output, a plurality ofattachment ports suitable for receiving signals from data terminals orthe like, at least one medium access controller (MAC) for receivinggenerating and/or repeating signals on the PR and switching units forselectively wrapping the PR and SR, the steps comprising:receiving afailure frame from the PR; wrapping the PR to the SR at the PR outputand SR input when the failure frame contains a source addresscorresponding to the MAC of the nearest downstream concentrator whileconnecting the MAC output to the SR output and isolating the MAC outputfrom the PR output and isolating the SR output from the SR input; andrepeating the failure frame on the PR when the source address of thefailure frame is not the address corresponding to the MAC of the nearestdownstream concentrator.
 16. The method of claim 15 wherein theconcentrator has a switching unit for selectively isolating the MAC ofthe concentrator from the PR and the SR and wrapping the input of theMAC to the output of the MAC and wherein said method further comprisesthe steps of:isolating from the PR and the SR the MAC of theconcentrator when a failure frame is received which has a source addressof the MAC of the nearest downstream concentrator; wrapping the PR andthe SR of the concentrator by connecting the PR to the SR after the lastattachment port and before the MAC of the concentrator and connectingthe PR output to the SR input; testing the isolated MAC to determine ifthe MAC is the cause of the failure; then reconnecting the isolated MACif the testing determines that the MAC is not the cause of the failureby reconnecting the MAC to the PR and SR while maintaining the isolationfrom the PR output and the SR input and relocating the wrap of the PRand SR to after the MAC of the concentrator.
 17. The method of claim 16further comprising the steps of:reevaluating the PR and SR connectionafter a preselected time if the MAC of the concentrator was determinedto not be the cause of the failure by reconnecting the concentrator PRinput to the PR output and the SR output to the input and monitoring thePR to determine if a failure continues to occur; and disconnecting theconcentrator PR and SR from the PR output and SR input of theconcentrator, connecting the PR output to the SR input and connecting PRto SR subsequent to the MAC of the concentrator if the failure continuesto occur.
 18. The method of claim 15 wherein said failure frame is anIEEE 802.5 Type 1 beacon frame.
 19. The method of claim 15 wherein theMAC of the concentrator can generate signals on the PR, said methodfurther comprising the steps of:detecting if the failure has occurred onthe PR; generating the failure frame on the PR when said detecting stepdetects a failure and wherein the failure frame contains the address ofthe MAC of the concentrator; determining if the failure frame isreceived on the PR with the address of the MAC of the concentrator;bypassing the failure when the MAC generating the failure frame has notreceived the failure frame from the PR within a preselected time byconnecting a first attachment port to the SR input of the concentratorwhile isolating the SR output from the SR input and the PR input fromthe first attachment port and connecting the SR output to the PR input.20. A reconfiguration unit for a dual ring communication systemcomprising:a primary ring input; a primary ring output; a secondary ringinput; a secondary ring output; a plurality of attachment ports having aring in connection and a ring out connection suitable for receivingsignals from data terminals and the like; a medium access control unit(MAC) having a ring in connection and a ring out connection wherein saidMAC is connected downstream of a last active attachment port forreceiving, generating and/or repeating signals; port reconfigurationmeans connected to said primary ring input, said primary ring output,said MAC and said plurality of attachment ports for selectivelyconnecting primary ring input, primary ring output, said MAC and saidattachment ports to form a ring path between active attachment ports andsaid MAC in response to control signals; control means connected to saidMAC and said port reconfiguration means for providing said controlsignals to said port reconfiguration means and for receiving andtransmitting signals to said MAC; and downstream wrap means responsiveto said control means for selectively disconnecting said SR input fromsaid SR output, disconnecting said PR input from said ring in connectionof a first active attachment port and for connecting said ring inconnection of said first active attachment port to said SR input andconnecting said PR input to said SR output.
 21. The reconfiguration unitof claim 20 further comprising:internal ring connection means responsiveto said control means for selectively connecting the ring in and ringout connections of the active attachment ports to the ring in and ringout connections of said MAC to form a ring of said active attachmentports and said MAC internal to said reconfiguration unit.
 22. Thereconfiguration unit of claim 20 further comprising:MAC isolation meansresponsive to said control means for selectively isolating said MAC fromsaid attachment ports and said input and output ports and for connectingsaid ring in connection to said ring out connection of said MAC whileconnecting a PR output port to a SR input port and connecting the ringout connection of the last active attachment port to said SR outputport.
 23. The reconfiguration unit of claim 21 furthercomprising:reconfiguration unit isolation means responsive to saidcontrol means for selectively activating said internal ring connectionmeans and for connecting said PR output to said SR input and said PRinput to said SR output.
 24. The reconfiguration unit of claim 20further comprising:upstream wrap means responsive to said control meansfor connecting said PR output to said SR input and connecting said ringout connection of said MAC to said SR output.
 25. A dual ringcommunication system capable of fault isolation and fault bypasscomprising:at least two reconfiguration units connected so as to form aring wherein at least one of said reconfiguration units is a singlemedium access controller (MAC) reconfiguration unit, and where saidsingle (MAC) reconfiguration unit comprising: a primary ring input; aprimary ring output; a secondary ring input; a secondary ring output; aplurality of attachment ports having a ring in connection and a ring outconnection suitable for receiving signals from data terminals and thelike; a medium access control unit MAC having a ring in connection and aring out connection wherein said MAC is connected downstream of a lastactive attachment port for receiving, generating and/or repeatingsignals; port reconfiguration means connected to said primary ringinput, said primary ring output, said MAC and said plurality ofattachment ports for selectively connecting input ports, output ports,said MAC and said attachment ports to form a dual ring communicationsystem between active attachment ports, said MAC and said inputs andoutputs in response to control signals; control means connected to saidMAC and said port reconfiguration means for providing said controlsignals to said port reconfiguration means and for receiving andtransmitting signals to said MAC; and downstream wrap means responsiveto said control means for selectively disconnecting a SR input from a SRoutput, disconnecting said PR input from said ring in connection of afirst active attachment port and for connecting said ring in connectionof said first active attachment port to said SR input and connectingsaid PR input to said SR output.
 26. The dual ring communication systemof claim 25 wherein each of said reconfiguration units is a single MACreconfiguration unit.